1. Field of the Invention
The present invention relates in general to the field of a sensor network, and more specifically to a communication protocol between sensor nodes.
2. Description of the Related Art
A sensor network is a core technical infra configured to realize ubiquitous computing technology and may be operated by wire or wireless connection. Since respective nodes in the sensor network, especially in wireless networks, use a limited capacity of battery, a main problem of the sensor network is to transmit sensing data using the minimum energy. Further, network latency must be reduced to the minimum in a large sensor network for the purpose of applying the sensor network to a real-time emergency service. Accordingly, the sensor network necessary for real-time monitoring requires a protocol with a very low network latency or delay while minimizing battery consumption.
A node of a wireless sensor network is operated by using a battery for a sensing operation, a computing operation. Problems such as pack collision, overhearing, control packet overhead, idle listening and the like, which energy is wasted in a wireless network, are arisen in case that a conventional MAC protocol such as an existing MANET and IEEE802.11 is applied to the sensor network. Specifically, the idle listening consumes most of energy, according as a sensor node is always operated in active state even in an interval of which a communication function is not required. In order to avoid above disadvantage, the conventional MAC protocol used in the sensor network reduce energy consumption by being operated in an active state by being awakened periodically while being ordinarily operated in a sleep state. As an example of such conventional MAC protocol, a sensor MAC protocol is described as follows.
<Sensor-MAC (S-MAC)>
The S-MAC protocol converts periodically a sensor node into a sleeping mode with a low duty cycle so as to reduce idle listening, a main cause of the energy consumption in the wireless network. This operation may reduce energy consumption as a main problem of the sensor network, get extensibility and avoid a packet collision, using competition-based scheduling. However, it has a disadvantage of increasing network latency due to the idle listening.
<Timeout-MAC (T-MAC)>
The T-MAC is a MAC protocol for a wireless sensor network based on competition such as the S-MAC. The T-MAC also reduces energy consumption by applying an active/sleep duty cycle proposed in the S-MAC. However, according as the duty cycle proposed in the S-MAC is applied in a fixed form, the efficiency of energy saving is reduced due to a change of traffic environment. In order to improve the disadvantage, the T-MAC is made to flexibly operate an active period according to data traffic. The T-MAC may increase the efficiency of energy saving due to the change of traffic environment by reducing idle listening less and less. However, similarly to the S-MAC, a problem of increasing network latency is still remained due to the idle listening.
<B-MAC>
The B-MAC is a MAC protocol basically based on CSMA. The MAC protocol uses a low power listening (LPL) for reducing idle listening to the minimum. A node configured to periodically perform listening for a very short time using the LPL and transmit data with packet having a preamble longer than the interval. When the preamble is recognized in an interval of the LPL, the node receives the following data. The B-MAC also generates a lot of energy consumption.
The above disadvantages of the conventional MAC protocol can be summarized as follows.
<Aspect of Energy Efficiency>
The S-MAC using the fixed duty cycle is successful in an aspect of general energy efficiency in comparison with the other conventional wireless MAC protocol. However, the energy efficiency of the S-MAC is greatly decreased in case of the node senses a very small volume of data in the sensor network, because of an unnecessary idle listening. In order to improve the disadvantage in the S-MAC, the T-MAC selects a method for saving energy by operating a timer, in case that a traffic environment of a network is lowered, by using a flexible duty cycle and reducing a unnecessary idle listening time, but consumes energy as much as the time wasted for setting a separate timer. And since the above duty cycle can not be adopted in an environment that a volume of network traffic is very high, it is difficult to effectively obtain an effect of energy saving.
The S-MAC or the T-MAC and B-MAC additionally consumes energy for transmitting or receiving a control packet or a long preamble except that energy is basically consumed to transmit data, and also consumes energy through idle listening for a specific interval depending on the traffic environment. Additionally, the S-MAC and the T-MAC generate additional energy consumption by using a CSMA-CA type, and the B-MAC additionally generates energy consumption due to overhearing.
<Aspect of Network Latency>
An existing MAC protocol for sensor network has a very big disadvantage in network latency. In case that a duty cycle is decreased to increase the energy efficiency of the S-MAC, the T-MAC and the B-MAC as protocols for sensor network, the network latency is increased in proportion thereto. In other words, in the existing conventional protocols, the energy consumption is remained to be inversely proportional to the network latency. This arises a severe problem in case that a real-time application is needed while using the limited power supply such as the battery. Specifically, the network latency is seriously affected in case that the sensor networks are in large scale.
<Problem of High Priority Packet Processing>
In order to use the wireless sensor network in emergency situation monitoring, the sensed data are required to be transmitted according to high priority. In other words, when data to be transmitted are in case of emergency, the emergency data must be given priority to non-emergency data. As such, a transmission chance must be given to a node having a higher priority. However, the above-described conventional communication type does not show a method for giving a priority in transmission.
<Aspect of Reliability>
The reliability has become a very important requirement while a technology of a sensor network is advanced to application stages. Specifically, in case that the technology of the sensor network is applied to emergency situation monitoring, the reliability becomes the most important issue. The existing conventional sensor network protocol just has considered reliability as a transmission issue. However, in order to assure the reliability, it is also needed to monitor the sensor nodes are in an error or are not operated.